1. Field of the Invention
The present invention relates to a method of manufacturing a cylinder block for a multi-cylinder engine that has a crankcase in which a plurality of crank chambers are aligned with corresponding cylinders in the direction of a crankshaft and the adjacent crank chambers are separated by partition walls. Particularly, the present invention relates to a method of manufacturing a cylinder block for a multi-cylinder engine, whereby ventilating communication holes are suitably formed in the partition walls so that the crank chambers may communicate with one another.
2. Description of Related Art
A conventional crankcase having a cylinder block for a multi-cylinder engine is designed such that a plurality of adjacent crank chambers are aligned with corresponding cylinders in the direction of a crankshaft. Partition walls separating the adjacent crank chambers usually have ventilating communication holes such that the crank chambers communicate with one another. This may help reduce friction by eliminating the problem of changes in internal pressure in any of the crank chambers arising from the reciprocating motion of corresponding pistons increasing with engine speed (refer to Japanese Patent Application Laid-Open No. 2000-136752).
Generally, a cylinder block for a multi-cylinder engine is a Siamese cylinder block designed such that part of each cylinder wall is shared by adjacent cylinder bores. Therefore, each cylinder wall between adjacent cylinder bores is thinner than each partition wall formed in the crankcase. In order to prevent a tool used for honing the cylinder bores from interfering with the partition walls, the cylinder-bore side of each partition wall has a honing runoff portion that is continuous with the corresponding cylinder bore. In other words, each partition wall includes a thin honing runoff portion and a thick partition wall portion extending below from the honing runoff portion via a step.
However, the communication holes defined by thick partition walls may affect the support strength of a bearing provided for a crankshaft. To avoid this, the communication holes may be formed in the honing runoff portions located away from the bearing for the crankshaft. On the other hand, because the larger communication holes reduce the passage resistance of the communication holes, the communication holes are formed across the corresponding thin honing runoff portions and thick partition walls. A machine may be used to form such larger communication holes in the partition walls of a cast cylinder block, parallel to the central axis of the crankshaft. However, the machine may require boring through not only the partition walls with the honing runoff portions, but also the portions including the steps. In this case, the tool may strike against the thick partition walls, thereby decreasing the processing accuracy and yield for the communication holes. In addition, boundaries between the corresponding steps and communication holes form edges, where stress may tend to concentrate. These stress concentrations may limit the size of the communication holes.